This invention relates to semiconductor integrated circuit devices, and more particularly to integrated circuits having both digital and analog functions on the same chip.
In digital integrated circuit devices such as microprocessors or memory, analog circuits are sometimes needed to achieve functions not realizable through strictly digital means. For example, DRAM devices have used temperature compensation circuits with bipolar devices to implement internal power supply generators and slew rate control circuits.
Analog functions, however, are particularly susceptible to noise which is prevalent in large digital chips. This impediment has been prohibitive to the use of analog circuits to enhance microprocessor performance. For example, phase locked loops would be quite useful to control clock slew on microprocessor chips, where component density has increased and clock distribution has become more difficult.
The digital designer has avoided utilizing analog circuits in digital chips, however, mainly because these circuits tend to malfunction in noisy "digital" environments. The stability of feedback loops (as used in phase locked loops) is deteriorated when noise rejection is a design requirement.
Previously, attempts have been made to alleviate the problems of using analog circuits on digital chips. For example, in U.S. Pat. No. 4,857,770, for "output Buffer Arrangement for Reducing Chip Noise Without Speed Penalty" issued to Hamid Partovi, a method was disclosed which used routing an analog level to different localities, with capacitive coupling to the appropriate rail. In U.S. Pat. No. 4,628,343, for "Semiconductor Integrated Circuit Device Free From Mutual Interference Between Circuit Blocks Formed Therein" a device uses an isolation region between different circuit blocks, with connection between blocks being by way of a shielded contact hole in conductor layer having substantially zero impedance.